1. Field of the Invention
The present invention relates to a recording sheet using toner such as a recording sheet for electrophotography and a process for thermally fixing a toner image formed thereon, and particularly relates to a recording sheet and a process for thermally fixing capable of preventing quality of a toner image from being degraded despite of no oil supplied to a fixing means. The present invention is preferably applicable to a OHP film having a good transparency and a thermally fixing process thereof.
2. Description of the Related Art
In a general electrophotography system, a toner image is formed on a recording sheet by transferring toner, and the toner image is then thermally fixed thereon by heating and pressing by means of a fixing means such as a fixing roller. The fixing roller is generally provided with a mechanism going on applying silicone oil thereto to reduce affinity of the toner to the fixing roller for the purpose of preventing offset of the toner itself.
When the recording sheet is used to form a colored image, and particularly adapted for an OHP(i.e., an over head projector) system, a receptor layer having a good affinity to the toner is generally formed on a plastic substrate film of the recording sheet in order to improve transparency of the toner image. Offset of the receptor layer itself owing to the thermal fixing is also prevented by application of the silicone oil.
A recording sheet for OHP has recently used as a preferable communicative medium on various occasions such as a lecture, a school, a company, an explanatory meeting, an exhibition or the like, and wish for a high quality of an image has grown stronger. Wish for an oil-less fixing system is then growing stronger because of unpleasantness of the silicone oil adhering to the recording sheet for OHP, degradation of the image owing to the silicone oil, an environmental issue or the like.
When a conventional recording sheet for OHP having a receptor layer essentially formed of a low molecular thermoplastic resin is used in the oil-less fixing system, the oil-less fixing system however causes sticking or fusion of the receptor layer of the recording sheet to the fixing roller or cohesive failure of the receptor layer itself, thus involving the recording sheet in deterioration of transparency.
There has been made many proposals for the toner, the recording sheet and the fixing process in order to realize the oil-less fixing process.
Japanese Patent Application Laid-open (KOKAI) Hei 9(1997)-218527 proposes that a receptor layer on a substrate contains a thermoplastic resin and a release agent having a melting point in a range of 40-120.degree. C. It is however difficult for such a proposed recording sheet to prevent cohesive failure of the receptor layer because the thermoplastic resin in the receptor layer is easily softened in order to improve quality of the image.
Japanese Patent Application Laid-open (KOKAI) Hei 9(1997)-171265 proposes that a flow beginning temperature of a binder resin in a receptor layer is set to 0-10.degree. C. higher than that of a binder resin in toner, and an angle made by an inclination of a protruding portion of a toner particle lodges in the receptor layer is set to 40 degrees or less with respect to the receptor layer. Though this prior art discloses that a surface temperature of a fixing roller should be set within a range of 130-160.degree. C., the receptor layer is actually liable to be caused offset in this range of the surface temperature.
Japanese Patent Application Laid-open (KOKAI) Hei 9(1997)-160278 proposes a receptor layer containing a thermoplastic resin and a release agent selected among silicone oil and fluorine type oil. It is however difficult for such a proposed receptor layer to provide a high quality of an image because toner embedded in the receptor layer is not compatible with the release agent.
Japanese Patent Application Laid-open (KOKAI) Hei 9(1997)-152736 proposes a receptor layer containing a thermoplastic resin and a plasticizer. Such a proposed receptor layer is however involved in the same problem of compatibility.